1. Field of the Invention
The present invention relates to a ballast circuit. More specifically, the present invention relates to a ballast circuit capable of detecting and automatically restarting all lamps.
2. Descriptions of the Related Art
Lamps have found more and more widespread applications since the advent thereof. One of the most widespread and well-known applications is lighting. As increasingly heightened requirements are being imposed on the lighting effect, ballasts with a plurality of lamps have been used wider than that with one lamp.
Please refer to FIG. 1, where a conventional ballast circuit 1 is illustrated. The ballast circuit 1 comprises a first lamp set 11, a second lamp set 12, a drive circuit 13, capacitors 150, 151, switches 152, 153, a detection circuit, a latch circuit 140, and preheat circuits 160, 161. The lamp set 11 comprises a resonant module 110, preheat circuits 111, 112 and two containing areas (not shown) for receiving the lamps 113, 114 respectively. The second lamp set 12 comprises a resonant module 120, preheat circuits 121, 122 and two containing areas (not shown) for receiving the lamps 123, 124 respectively. The drive circuit 13 is configured to start the switches 152, 153 of the ballast circuit 1. Various elements of the ballast circuit 1 are well known to those of ordinary skill in the art, and thus only those relevant to this invention will be described.
As the number of lamps contained in the ballast circuit 1 increases, the lighting effect gets significantly improved. However, there are some shortcomings arisen in the ballast circuit containing a plurality of lamps. One of the shortcomings is that not all lamps can be detected whether they have been connected into the circuit. More specifically, the detection circuit of the ballast circuit 1 comprises a capacitor 141 and resistors 142, 143, 144. The resistor 142 is connected to a direct current (DC) voltage input terminal 163 and a filament of the lamp 114. The resistor 143 is connected to a filament of the lamp 124 and the latch circuit 140. The resistor 144 is connected to the resistor 143 and the latch circuit 140.
As the detection circuit is connected to the lamps 114, 124 and the latch circuit 140, the latch circuit 140 may determine whether the lamps 114, 124 have been connected into the circuit according to a voltage at a juncture 17. More particularly, in case either or both of the lamps 114, 124 fail to be connected into the circuit, the juncture 17 will experience a voltage drop so that the ballast circuit 1 can determine that the lamp(s) has not been connected into the circuit. Once all the lamps have been connected into the circuit, the voltage at the juncture 17 will be restored to a normal value, and the drive circuit 13 can restart the lamps.
It follows from the above descriptions that the detection circuit equipped in the ballast circuit 1 is only able to detect whether a portion of the lamps (i.e., lamps 114, 124) has been connected to the circuit or not, but fails to detect whether all of the lamps (i.e., fails to detect the lamps 111, 121) have been connected to the circuit or not. Consequently, the aforementioned ballast circuit is inconvenient for users.
In summary, efforts still have to be made in the art to provide a ballast circuit capable of detecting if all lamps have been connected into the circuit.